RF power amplifiers (RF PAs), are well known in the art. Such devices may be fabricated in the form of a single discrete device containing a single amplification device (e.g. a transistor die) and associated compensation devices (e.g. a MOS capacitor) (FIGS. 1 and 2).
Within the RF PA, an RF power amplifier die (10) is, typically, mounted to a metal (e.g. gold plated kovar or tin plated copper) collector lead (11) which is, in turn, mounted to an electrically non-conductive beryllium oxide (BeO) pedestal (16). The BeO pedestal, in turn, supports a base lead (12), and an emitter lead (13) which are wire-bonded to the die. The BeO pedestal may, in turn, be attached to a heat conductive, mounting support (14). The support (14), together with a cover (15), encloses the die and wire-bonds; and between which the collector, emitter and base leads project, provides an RF PA enclosure.
Cooling of the die occurs primarily through transfer of heat from the die into the metal leads (the lead frame) (11, 12, and 13). Cooling of the lead frame (11, 12, and 13), in turn, occurs through conduction into the pedestal (16). Heat passing into the pedestal is then re-transmitted into the mounting support (14) and, consequently, into a heat sink (not shown).
The cover enclosing the die also conducts a small amount of heat from the die, convectively, into the surrounding air. However, because the cover is in contact with the lead frame, it is typically constructed of an electrically non-conductive material, such as alumina (Al.sub.2 O.sub.3). Use of an alumina cover results in a poor rate of heat transfer through the cover (the thermal conductivity of alumina is 15-20 W/m.degree.K.). Because of the poor rate of heat transfer of alumina, the prior art alumina covers may be regarded as non-conductive in terms of heat transfer from collector lead to support (14).
The pedestal (16), through which most of the heat from the die passes, has a fairly high rate of thermal conductivity (230-260 W/m.degree.K.). During periods of continuous transmission, on the other hand, the rate of heat transfer through the pedestal (16) may not match the rate of thermal output of the die (10). The die (10) and collector lead (11), in such cases, may overheat and cause damage to the die (10) or to associated solder joints.
Past efforts at improving cooling of RF PAs have included attachment of fins or heat sinks to the collector lead external (11) to the RF PA enclosure. While these efforts have provided some protection from some types of short term overload, damage may still occur during periods of continuous transmission. Because of the importance of RF PAs a need exists for improved methods of cooling RF PAs.